Novolak cyanate-based prepolymer compositions

ABSTRACT

Described are prepolymer compositions on the basis of novolak cyanates of the general formula  
                 
 
     wherein n is a number from 0 to 20 and the radicals R are identical or different and represent hydrogen or methyl. The compositions contain:  
     30 to 90 parts by weight of a prepolymer from novolak cyanates (I) wherein R=hydrogen (Ia) and 10 to 70 parts by weight of novolak cyanates (I) wherein R=methyl (Ib), and optionally highly disperse silicon dioxides and/or particulate or fibrous fillers. The inventive compositions are solid at normal temperature and can be ground to flowable powders. They have excellent stability when stored and are easy to process and especially suitable for use as the resin component in base materials for printed circuits on resin-bound abrasive products.

DESCRIPTION

[0001] The invention relates to prepolymer compositions based on novolak cyanates.

[0002] Novolak cyanates are reactive oligomeric compounds of the general formula

[0003] in which n is generally a number from 0 to 20 and the radicals R are identical or different and represent hydrogen or methyl. The bonds to the methylene groups which link the benzene rings can in principle go out to the cyanate groups from the ortho-(o), meta-(m) or para-(p) positions and the radicals R can be located in any of the remaining positions. Linking preferably takes place through the ortho and para positions. The compounds are customarily in the form both of oligomer mixtures (with different values of n) and of isomer mixtures (with different linkage patterns, preferably o or p for the terminal benzene rings and o,o or o,p for the nonterminal benzene rings) and are obtainable from the condensation products of phenol or cresols with formaldehyde by reaction with cyanogen chloride or bromide. The oligomer mixtures are customarily characterized by stating the average value of n. This average value—unlike the value of n in one individual molecule—can also be nonintegral.

[0004] Novolak cyanates (I) with R=hydrogen or R=methyl are available commercially, for example, under the designation Primaset® from the company Lonza AG, Basle, Switzerland.

[0005] The novolak cyanates can be polymerized thermally, optionally in the presence of catalysts, to form polytriazines, with three cyanate groups in each case undergoing cyclotrimerization to form a 1,3,5-triazine ring. This cyclotrimerization is strongly exothermic. Owing to the outstanding thermal, mechanical, and electrical properties of polytriazines, novolak cyanates are being used to an increasing extent, alone or in a mixture with other cyanates, for producing high-temperature-resistant moldings.

[0006] In order to improve the processing properties and to lower the exothermic heat in the course of curing, it is advantageous for certain applications to carry out the cyclotrimerization in part even before the shaping operation, to give a—preferably solid—“prepolymer”, which still has free cyanate groups, optionally is soluble in organic solvents, and after the shaping operation can be cured to completion and crosslinked with reaction of the remaining cyanate groups. The prepolymers from the prior art, however, also have negative properties, in that they are tacky and are neither readily grindable nor free-flowing or, owing to excessive crosslinking, are no longer shapeable or do not have the desired reactivity.

[0007] An object of the present invention was therefore to provide novolak cyanate prepolymers which are solid at standard temperature and meltable at a higher temperature, can easily be ground to a readily free-flowing powder, and are neither too reactive nor too inert. In particular they should be storable for a long time at standard temperature, should have a gel time of at least 6 minutes at 200° C., but after gel formation should quickly cure fully.

[0008] In accordance with the invention this object is achieved by the prepolymer compositions of claim 1.

[0009] It has been found that defined mixtures of prepolymerized and nonprepolymerized novolak cyanates, specifically mixtures of from 30 to 90 parts by weight of prepolymers of novolak cyanates of the general formula

[0010] in which n is a number from 0 to 20 and the radicals R represent hydrogen (Ia), with from 10 to 70 parts by weight of novolak cyanates (I) in which R in each case represents methyl (Ib), have the desired properties. The sum of the parts by weight of prepolymer and novolak cyanate (Ib) in this composition amounts in each case to 100.

[0011] The compositions of the invention preferably contain from 50 to 70 parts by weight of prepolymers of novolak cyanates (Ia) and from 30 to 50 parts by weight of novolak cyanates (Ib).

[0012] The average value of n in the novolak cyanate (Ia) from which the prepolymer has been obtained is preferably a number from 0 to 5.

[0013] Particularly good results are achieved in this context with prepolymers having a refractive index n_(D) ⁹⁸ (refractive index for the Na D-line at 98° C.) of at least 1.58, preferably at least 1.582.

[0014] Likewise preferred are prepolymer compositions in which the average value of n in the novolak cyanates (Ib) is from 1 to 10.

[0015] In one preferred embodiment the prepolymer compositions of the invention further contain up to 5% by weight of a highly disperse silica. As highly disperse silica it is possible to use both pyrogenic (e.g., Aerosil®) and precipitated (e.g., Sipernat®) products. Particular preference is given to pyrogenic silicas.

[0016] The prepolymer compositions of the invention are suitable in particular as a resin component in base materials for printed circuit boards or syntheticresin-bound abrasive products.

[0017] The examples which follow illustrate the execution of the invention without representing any restriction.

[0018] The gel times were determined at 200±1° C. in analogy to DIN 16 945 using Gelnorm® instruments (Gel Instrumente AG, CH-8800 Thalwil) on samples of approximately 12 g in conventional test tubes (16×160 mm). The duration of one up-down cycle was 10 s. The refractive indices were determined using an Abbe refractometer thermostated at 98° C.

[0019] The grinding experiments were conducted in a vibratory mill with ceramic grinding beads and grinding drums. The starting materials used were Primaset® PT-30 (Ia, n≈3), a yellow resin of high viscosity with n_(D) ⁹⁸=1.5667 and a gel time of 2.5 h, and Primaset® CT-90 (Ib, n=1-10), a yellow amorphous powder having a gel time of 0.5 h. Both products are available from Lonza AG, Basle, Switzerland.

COMPARATIVE EXAMPLE 1

[0020] Prepolymers from Primaset® PT-30

[0021] Primaset® PT-30 was heated at 120° C. for 30-40 min for liquefaction and in portions each of 10-20 g was poured into disposable aluminum trays (Ø=60 mm). The prepolymerization was conducted at 165° C. for times of 1-9 h. The key properties of the samples thus obtained are collated in table 1 below.

[0022] It was found that with Primaset® PT-30 alone the desired properties cannot be achieved: either the prepolymer is ungrindable or sticks together or it has already undergone crosslinking to such an extent that further processing is no longer possible. TABLE 1 Free- Gel flowability t time Appearance Appearance Free- after [h] n_(D) ⁹⁸ [min] [RT] (165° C.) Grindability flowability 3′/50° C. 1 1.5696 — yl-rd, highly — — — honeylike mobile 2 1.5714 — yl-rd, highly — — — honeylike mobile 3 1.5738 — yl-rd, highly — — — honeylike mobile 4 1.5744 — or-bn, highly — — — flows mobile 5 1.5780 — or-bn, highly — — — flows mobile 5½ 1.5809 24 or-bn, highly — — solid, mobile plastic 6 1.5810 19 or-bn, liquid — — solid, plastic 6½ 1.5846 14 or-bn, liquid poor none completely solid, (tritu- melted plastic rated with dry ice) 7 1.588*  7 or-bn, viscous grindable with sticks solid tapping together 7½ not or-bn, viscous readily good sticks measurable solid grindable together 8 (cannot be or-bn, solid readily good good after liquefied) solid grindable tapping 8½ or-bn, solid readily good good after solid grindable tapping 9 or-bn, solid n.m. n.m. n.m. solid

COMPARATIVE EXAMPLE 2

[0023] Prepolymers From Mixtures of Primasete PT-30 and Primaset® CT-90 (Joint Prepolymerization)

[0024] The procedure described in comparative example 1 was repeated but using instead of Primaset® PT-30 alone mixtures of Primaset® PT-30 and Primaset® CT-90 in proportions of 90:10, 70:30, 50:50, 30:70, 20:80 and 10:90. The prepolymers obtainable in this way were again either not grindable or free-flowing or had already undergone excessive crosslinking.

EXAMPLE 1 Inventive

[0025] The procedure described in comparative example 1 was initially repeated but after the polymerization at 165° C. the samples were cooled to 120° C. and at this temperature were mixed with Primaset® CT-90 so as to form in each case a liquid homogeneous mixture. The refractive index of these mixtures and, after cooling to standard temperature, their grindability were determined. A measurement was made of the gel time as well. Good results were obtained with prepolymerization times of 6¼ and 6½ h. The key results are collated in table 2 below. TABLE 2 Ib (parts Gel Free- by t time Appearance Grindability flowability weight) [h] [min] (RT/165° C.) at RT at RT  0*) 6¼ 14 solid/sluggishly — — mobile 10 6¼ 12 solid/sluggishly good Good mobile 20 6¼ 14 solid/sluggishly good Good mobile 30 6¼ 16 solid/sluggishly good Good mobile 50 6¼ 14 solid/sluggishly good Good mobile  0*) 6½ 15 solid/sluggishly — — mobile 10 6½ 16 solid/sluggishly limited poor, with mobile tapping 20 6½ 18 solid/sluggishly good poor, with mobile tapping 30 6½ 17 solid/sluggishly good Good mobile 50 6½ 15 solid/sluggishly good Good mobile

[0026] The samples exhibited the desired temperature behavior (solid at room temperature, sluggishly mobile at 165° C.) and an advantageous gel time. In contrast to the non-inventive samples (0 parts Primaset® CT-90), all were grindable and after grinding were free-flowing. By adding 1% of highly disperse silica (Aerosil® R972, Degussa-Hüls) it was possible to achieve a further significant improvement in free-flowability. 

1. Prepolymer compositions based on novolak cyanates of the general formula

in which
 2. Prepolymer compositions of claim 1, characterized in that the average value of n in the prepolymer (Ia) is from 0 to
 5. 3. Prepolymer compositions of claim 2, characterized in that the prepolymer of the novolak cyanate (Ia) used has a refractive index n_(D) ⁹⁸ of at least 1.58, preferably at least 1.582.
 4. Prepolymer compositions of one of claims 1 to 3, characterized in that the average value of n in the novolak cyanates (Ib) is from 1 to
 10. 5. Prepolymer compositions of one of claims 1 to 4, characterized in that they further contain up to 5% by weight of a highly disperse silica.
 6. Prepolymer compositions of claim 5, characterized in that as highly disperse silica they comprise a pyrogenic silica.
 7. Prepolymer compositions of one of claims 1 to 4, characterized in that they further comprise at least one particulate and/or fibrous filler.
 8. Use of the prepolymer compositions of claims 1 to 7 as a resin component in base materials for printed circuit boards or synthetic resin-bound abrasive products. 